This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The broad goal of the project is to determine age-related changes in the physiology of the hearing end organ (the cochlea) in the common marmoset (Callithrix jacchus) by using non-invasive procedures. The procedure measures sounds generated by the cochlea (otoacoustic emissions or OAEs) in response to acoustic stimulation of the ear. OAEs are presumed to be generated by the activity of outer hair cells which are important for improving hearing performance. With hearing loss, hair cells are damaged or unable to function effectively, and this causes a reduction in OAEs. Thus, OAEs can provide a direct measure of cochlear status, in particular the status of outer hair cells. The immediate goal of the project is to ascertain whether marmosets demonstrate OAEs that are similar to humans and Old World primates, and whether they are affected by aging. The long-term goal is to carry out invasive physiological studies and to develop prosthetic implants for treating hearing loss. Specific Aims: The two specific aims of the project are (1) to determine the effects of hearing on normative evoked otoacoustic emissions, and (2) the effect of aging on OAEs when the contralateral ear is stimulated. We proposed to evaluated two types of evoked OAEs: distortion product (DP) and click-evoked (CE) OAEs in four different age groups. Because OAEs are sensitive to the stimulation parameters, we will first determine the most optimal stimulation parameters for eliciting the strongest OAEs, and ascertain whether these parameters are similar to humans and Old World primates. Two additional procedures were added in the current reporting year. 1) Measurement of auditory brainstem responses (ABRs). This is a non-invasive measurement of gross evoked potentials recorded using subdermal electrodes. These evoked potentials are measured in response to sound stimulation. The recorded potentials are analyzed for peak amplitude and latency and can provide information on thresholds. 2) Passive recording of vocal communication and vocal interaction within a colony of caged marmosets. The animals are monitored with microphones, and the data is analyzed for interactions.